When making dental prosthesis or parts thereof, it is customary to utilize as base material cast or sintered alloys of various types, such as precious metal alloys having a gold or palladium base, or other alloys, particularly having a nickel or cobalt base. These metallic base materials are then coated, at least in part, with a dental porcelain or similar ceramic so that the appearance of the prosthesis will simulate natural teeth, or match other teeth. The base materials on which the dental porcelain is applied must meet various specification.
The necessary strength required for dental prosthesis is obtained by adding to alloys based on gold various metals of the platinum group of the elements of the periodic system, and non-precious metals such as indium, tin, and iron, in selected proportions. The non-precious metals are believed to favor adhesion of dental porcelain by formation of oxides.
It is customary, when making a dental prosthesis, to first make the base prosthesis of the desired metal by a casting or sintering process. The metallic prosthesis is then exposed to firing in an oxidizing atmosphere, at normal or reduced pressure, and at a temperature of between 950.degree. to 980.degree. C. The surface of the prosthesis then will have a layer of a non-precious metal oxide formed thereon, the layer having a thickness which is difficult to control. If the time of firing and oxidation is increased, the thickness of the oxide layer will likewise increase, and subsequent dental porcelain applied to the oxidized layer will not adhere well. The non-precious metal oxides have a dark coloration, and if the porcelain layer is thin, the teeth will appear grey. It has been proposed to apply a thin blending layer of blending gold to obtain a gold-colored surface of the cast or sintered prosthesis when made with a gold-based alloy, and simultaneously to increase the adhesive force of the dental porcelain thereto. The blending gold is applied on the cast or sintered metallic dental prosthesis before application of the dental porcelain thereto.
Commerically available blending golds are highly temperature sensitive. If the furnace temperatures are too low, that is, are below about 1060.degree. C., the metallic dental prosthesis will have only a matte or dull yellow surface which is discontinuous. An increase in firing temperature leads to formation of drops of the gold layer, that is, the gold runs together to form drops leaving surface areas of the dental prosthesis uncovered in the spaces between the drops and, due to the formation of non-precious metal oxides, leaving a dark coloration.